In response to an increasing demand for large-volume data transmission, digital coherent optical transmission systems have been studied and developed to enable transmission at 100 Gbps or faster with single wavelength light, for example. Unlike an intensity modulation system, a digital coherent optical transmission system uses not only optical intensities but also optical phases in signal modulation. An example of such a modulation method is quadrature amplitude modulation (QAM). By QAM, the amplitudes of respective signals of an in-phase component and a quadrature-phase component are adjusted.
In a modulation system of this type, a skew, a power difference, and a quadrature deviation (phase rotation) are generated between a signal of an in-phase component and a signal of a quadrature-phase component in each transmitter and each receiver. This phenomenon is called IQ imbalance (also IQ unbalance, IQ incompleteness, or the like), and causes signal deterioration. To counter this, Japanese Patent Application Publication Nos. 2009-147498 and 2012-85302 (hereinafter, referred to as Patent Documents 1 and 2, respectively), for example, disclose IQ imbalance compensation means.